The deposition of metals onto surfaces has a long tradition in the art. This deposition can be achieved by means of electrolytic or electroless plating of metals. Even though these plating techniques have been used for many decades there are still many technical challenges unsolved. One such unresolved challenge is the deposition of metals into small cavities without producing too much over-plating.
The deposition of metals or metal alloys into recessed structures such as vias and trenches in the manufacturing of printed circuit boards, IC substrates and semiconductors is mostly achieved by using the so-called dual damascene process. Trenches and vias are etched into the dielectric prior to the deposition of barrier layers, typically nitrides of titanium or tantalum, followed by electrolytic copper filling of the recessed structures and subsequent chemical-mechanical planarization (CMP). Upon decreasing the size of such trenches and vias, however, high plating rates result in too much over-plating of the deposited metal which then have to be removed by a costly CMP and/or chemical etching step. This increases the number of process steps and the waste produced in the overall process, both of which is highly undesirable. Furthermore, electrolytic copper deposits often contain voids which increase the resistivity of interconnects.
An alternative to electrolytic deposition of metals is electroless plating thereof. Electroless plating is the controlled autocatalytic deposition of a continuous film of metal without the assistance of an external supply of electrons. Non-metallic surfaces may be pretreated to make them receptive or catalytic for deposition. All or selected portions of a surface may suitably be pretreated. The main components of electroless metal baths are the metal salt, a complexing agent, a reducing agent, and, as optional ingredients, an alkaline, and additives, as for example stabilising agents. Complexing agents (also called chelating agents in the art) are used to chelate the metal being deposited and prevent the metal from being precipitated from solution (i.e. as the hydroxide and the like). Chelating metal renders the metal available to the reducing agent which converts the metal ions to metallic form. A further form of metal deposition is immersion plating. Immersion plating is another deposition of metal without the assistance of an external supply of electrons and without chemical reducing agent. The mechanism relies on the substitution of metals from an underlying substrate for metal ions present in the immersion plating solution. In the context of the present invention electroless plating is to be understood as autocatalytic deposition with the aid of a chemical reducing agent (referred to a “reducing agent” herein).
In order to adjust the properties of the electroless plating bath and the metal or metal alloy deposit to be formed when using such an electroless plating bath, additives are added to the electroless plating bath in order to improve the properties both the electroless plating bath and the formed metal or metal alloy deposit.
β-amino acids or amides derived therefrom as stabilising agents for electroless plating baths are known from WO 2011/003116. However, such β-amino acids do not alter the plating rate (see Application Example 1).
U.S. Pat. No. 7,220,296 B1 discloses a process for the electroless deposition of copper into recessed structures of integrated circuits to form interconnects. Additives such as polyethyleneglycols may be added to the disclosed electroless copper plating bath to more selectively deposit copper into the recessed structures. Although these additives are known to have levelling effects in electrolytic plating baths they do not have any substantial effect on the plating rate or stability of electroless plating baths (see Application Example 6). Also, such additives are only to improve the wettability of surface in accordance with the teachings of US 2005/0161338 in case of cobalt plating.
JP 2007-254793 teaches nitrogen-containing polymers made of monomers such as dicyandiamide, lysine and mono- or diallylamines to be suitable stabilising agents for electroless nickel plating baths. Also, US 2014/0087560 A1 discloses nitrogen-containing polymers such as polyvinylamines to be used in electroless deposition of nickel and cobalt. The latter plating baths are particularly suitable for forming barrier layers in recessed structures prior to electrolytic copper deposition thereon as the plating rates are reduced. The use of polymers containing high amounts of amines is not desirable because such polymers are highly hazardous to water and may result in discolouration of deposited metal layers.